The present invention relates generally to integrated circuits (ICs). More particularly, the present application relates to a method and apparatus for improved scanning electron microscope (SEM) inspection and analysis of patterned photoresist features utilized to fabricate ICs.
During integrated circuit (IC) fabrication, various surfaces involved therein are inspected and analyzed for a variety of reasons. For example, the dimensions of features provided on a given surface may be measured and/or their alignment with respect to other features may be analyzed. Features provided on a given surface may be inspected for uniformity, integrity and/or defects. A semiconductor substrate, photoresist feature, or a layer above the semiconductor substrate can be inspected.
The semiconductor substrate or a layer above the semiconductor substrate, collectively, a semiconductor wafer, may be inspected to determine whether further processing should continue, whether the wafer should be discarded, or whether an appropriate corrective measure should be taken before further processing of the wafer continues. In this manner, the likelihood of defects occurring during the IC fabrication process can be decreased or eliminated.
Various techniques can be utilized to inspect and analyze the wafer. Optical microscopes, scanning electron microscopes (SEMs), or laser-based systems may be utilized for inspection and measurement tasks. Some of the tasks require human involvement and others are fully automated so that human involvement is unnecessary.
Layers or surfaces which are present on the wafer only during the IC fabrication process (i.e., layers or surfaces which do not comprise the end product IC) are also commonly inspected. For example, layers of photoresist material can be inspected following development (after-develop-inspection or xe2x80x9cADIxe2x80x9d) to ensure that the pattern transfer process has been performed correctly and/or that the pattern is within specified tolerances. From such inspection, mistakes or unacceptable process variations associated with the layer of photoresist material can be identified and corrected since the layer of photoresist material has not yet been utilized to produce any physical changes to the wafer itself, such as, by doping, etching, etc. Defective layers of photoresist material can be corrected by stripping and reapplying a new layer of photoresist material on the wafer.
Critical dimensions of patterned features on a layer of photoresist material are commonly measured using an SEM inspection and analysis tool. This measurement task involves obtaining SEM images of the patterned features. The SEM inspection and analysis tool obtains SEM images of a given sample using an inspection electron beam, the inspection electron beam characterized by a low beam current (on the order of pA) and an accelerating voltage of approximately 300-1500 V. The sample is rapidly scanned by the inspection electron beam so as to obtain imaging data but not long enough to intentionally affect the sample.
The SEM inspection and analysis tool includes an electron gun, one or more lens assemblies, and photomultiplier detectors, all within a vacuum environment at approximately 10xe2x88x927 Torr. Electrons emitted from the electron gun, i.e., the inspection electron beam, are focused by the lens assemblies to form primary electrons that impinge on a sample to be imaged (e.g., the patterned layer of photoresist material). The interaction of the impinging primary electrons with the surface of the sample causes secondary electrons to be emitted from the sample. The secondary electrons are generated from the top portion of the sample, within a depth of approximately 50-60 xc3x85 from the top surface. These secondary electrons are collected by the photomultiplier detectors and comprise the imaging data from which SEM images are generated.
However, when the photoresist material is an organic-based photoresist material, SEM images of features patterned thereon are susceptible to poor image contrast, and this in turn may lead to erroneous critical dimension measurements. SEM images with degraded image contrast are caused by undesirable interaction of the primary electrons with the sample (e.g., the organic-based photoresist material). Instead of merely causing secondary electrons to be emitted from the organic-based photoresist material, the primary electrons may also cause volatile organic species to be emitted or outgassed from the organic-based photoresist material (i.e., the outgassing problem). These volatile organic species interact with and scatter the secondary electrons such that the secondary electrons that are collected by the photomultiplier detectors are distorted imaging data representative of the patterned features on the photoresist material. Consequently SEM images generated therefrom are less than ideal, such as, suffering from degraded image contrast.
Additionally, organic-based photoresist materials have a tendency to build up charge and/or heat from the impinging primary electrons (i.e., the charging and heating problems). Organic-based photoresist materials exhibit insulative properties and can build up charge and/or beat from the beam current of the primary electrons. Because the constituents comprising the organic-based photoresist material have varying insulative properties with respect to each other, charge and/or heat dissipation is also non-uniform and/or insignificant. When excessive charge and/or heat builds up within the material, structural or physical changes can occur such that patterned features may become permanently distorted and damaged. Hence, not only are the SEM images inaccurate but subsequent pattern transfer to underlying layers of the wafer is also adversely impacted. As features are lithographically patterned at ever decreasing dimensions, the outgassing, charging, and/or heating problems associated with SEM imaging of organic-based photoresist surfaces are becoming progressively worse.
Thus, there is a need for improved SEM inspection and analysis of patterned features on a layer of photoresist material. There is a further need for a process for reducing charging and/or heating problems associated with SEM imaging of organic-based photoresist materials. There is still a further need for a process for reducing undesirable outgassing problems associated with SEM imaging of organic-based photoresist materials.
One exemplary embodiment relates to a method of inspecting a surface associated with manufacture of an integrated circuit. The method includes providing an electron beam to the surface, and transforming at least a portion of the surface. The method further includes inspecting the surface using a scanning electron microscope (SEM). The transforming step occurs before the inspecting step.
Another exemplary embodiment relates to a patterned photoresist layer. The layer is configured to facilitate accurate critical dimension measurements of features thereon using a scanning electron microscope (SEM). The layer includes a treated region and an untreated region. The treated region comprises a top surface and side surfaces surrounding the untreated region. The treated region has at least one of a different electrical and material property relative to the untreated region.
Still another exemplary embodiment relates to a process for reducing the build up of at least one of charge, heat, and volatile species in a photoresist layer during scanning electron microscope (SEM) inspection. The process includes exposing the photoresist layer to a flood electron beam, and forming a shell in the photoresist layer in response to the flood electron beam. The photoresist layer includes at least one patterned feature having a top surface, side surfaces, and an untreated portion. The shell is comprised of the top surface and the side surfaces. The shell reduces the build up of at least one of charge, heat, and volatile species associated with at least one feature during SEM inspection.